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    SubjectRe: [RFC] Page table sharing
    On Tue, 19 Feb 2002, Linus Torvalds wrote:
    > On Tue, 19 Feb 2002, Daniel Phillips wrote:
    > > >
    > > > At that point you might as well make the TLB shootdown global (ie you keep
    > > > track of a mask of CPU's whose TLB's you want to kill, and any pmd that
    > > > has count > 1 just makes that mask be "all CPU's").
    > >
    > > How do we know when to do the global tlb flush?
    >
    > See above.
    >
    > Basically, the algorithm is:
    >
    > invalidate_cpu_mask = 0;
    >
    > .. for each page swapped out ..
    >
    > pte = ptep_get_and_clear(ptep);
    > save_pte_and_mm(pte_page(pte));
    > mask = mm->cpu_vm_mask;
    > if (page_count(pmd_page) > 1)
    > mask = ~0UL;
    > invalidate_cpu_mask |= mask;
    >
    > and then at the end you just do
    >
    > flush_tlb_cpus(invalidate_cpu_mask);
    > for_each_page_saved() {
    > free_page(page);
    > }
    >
    > (yeah, yeah, add cache coherency etc).

    It's a little worse than this, I think. Propagating pte_dirty(pte) to
    set_page_dirty(page) cannot be done until after the flush_tlb_cpus, if
    the ptes are writable: and copy_page_range is not setting "cow", so not
    write protecting, when it's a shared writable mapping. Easy answer is
    to scrap "cow" there and always do the write protection; but I doubt
    that's the correct answer. swap_out could keep an array of pointers to
    ptes, to propagate dirty after flushing TLB and before freeing pages,
    but that's not very pretty.

    Hugh

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